1. Field of the Invention
The present invention relates to an automatic braking system for a motor vehicle and, in particular, to an automatic braking system which detects a distance and a relative speed between a vehicle and an obstacle in front of the vehicle and automatically operates brakes of respective wheels in order to avoid contacting the obstacle.
2. Description of the Prior Art
The conventional automatic braking system is disclosed in Japanese Patent Publication No. 39-2565, Japanese Patent Publication No. 39-5668 and the like. The automatic braking system continuously detects a distance and a relative speed between a vehicle and an obstacle in front of the vehicle by an optical device and/or an ultrasonic device, and judges, based on the detected distance and relative speed, whether or not the vehicle has a possibility of contacting with the obstacle. As a result, when the vehicle is judged to have the possibility of contacting the obstacle, an actuator automatically operates brakes of respective wheels in order to avoid the contact with the obstacle.
On the other hand, a vacuum-type booster is known from Japanese Laid-Open Publication No. 1-127446. The vacuum-type booster can generate a large braking force by using a pressure difference between negative pressure made by an intake air of an engine and atmospheric pressure.
The vacuum-type booster could be applied to the above-mentioned conventional automatic braking system. However, when the vacuum-type booster does not have a predetermined negative pressure, the automatic braking system can not obtain enough braking force to avoid the contact with the obstacle in front of the vehicle.